Sodium dodecyl sulphate (SDS) polyacrylamide gel electrophoresis of the liver cytosol of euthyroid male rats revealed a prominent band (molecular weight, 26 000 daltons), designated Protein II, which was virtually absent in the cytosol of hypothyroid animals. Injection of 500 mug triiodothyronine (T3) per 100 g body weight resulted in a maximal increase in the level of Protein II, reaching 90% of the euthyroidal level 3 days after hormone administration. Concomitant studies with the liver mitochondrial enzyme alpha-glycerophosphate dehydrogenase (alpha-GPD) indicated that this T3 dose also resulted in a maximal enzyme response in this time period. Since we have estimated that 500 mug of T3 will saturate nearly all nuclear T3 binding sites, these results support the concept that the synthesis of both proteins is limited by nuclear binding. Protein II was absent in the liver cytosol of female rats but could be induced in ovariectomized female rats by androgens. Treatment of male rats with oestradiol resulted in disappearance of Protein II. Since administration of testosterone to hypothyroid male rats caused only a minimal increase in the amount of Protein II, the absence of the protein in hypothyroid animals was not due to androgen deficiency. Similarities in the molecular weight and the response to hormonal manipulation of Protein II and of the urinary alpha2uglobulin, previously reported by Roy (1973) raise the possibility that these proteins are the same. The high concentration of Protein II in male rat cytosol and the relative ease in its identification by SDS polyacrylamide gel electrophoresis make it a potentially useful model protein for the study of thyroid hormone action at the cellular level.